U.S. Pat. No. 4,362,819 discusses the current high interest in glasses demonstrating low transition temperatures, i.e., below about 350.degree. C., but which also exhibit good chemical durability, resistance to weathering, and glass stability (resistance to devitrification and/or phase separation). The text of the patent is directed primarily to glasses within the alkali metal aluminofluorophosphate composition system.
That alkali metal fluorophosphate glasses inherently display low transition temperatures has been recognized for many years. Nevertheless, those glasses have generally evidenced poor durability and resistance to weathering such as to render them useful only in special, protected applications. Hence, U.S. Pat. No. 4,362,819 discloses the utility of carefully controlled contents and ratios of fluoride and alumina levels to achieve improved properties. Research has been continuous to formulate other fluorophosphate-based glasses also manifesting low transition temperatures coupled with advantageous chemical and physical properties.
The facility of ZrO.sub.2 to improve the chemical durability of a glass composition is well known in the art. However, zirconium is a very complex cation which can exhibit three coordination numbers; viz., with a coordination number of six it functions as a glass former; with a coordination number of eight it behaves as an intermediate; and with a coordination number of seven in a fluorine environment it assumes a distorted octahedral geometry with the seventh fluorine ion lying in one face of the octahedron. Recently, zirconium has been asserted to comprise the primary glass forming cation in an all-fluoride system (Poulain, M. et al., "Verres Flores au Tetrafluorine de Zirconium Proprietes Optiques d'un Verre Dope au Nd.sup.+3 ", Materials Research Bulletin, 10, No. 4, pages 243-246, April. 1975). Other disclosures of zirconium fluoride glasses include U.S. Pat. Nos. 4,141,741 and 4,328,218.
Because the coordination structure of zirconium is so varied, an investigation was undertaken to study its behavior in different glass forming systems.